The present invention relates generally to rotary drum filters and more particularly to center external valve rotary drum filters.
Rotary drum filters are commonly used in the pulp and paper-making industry to separate wood pulp from its filtrate. Such filters include a rotary drum partially submerged in a tank of pulp slurry. The drum has axially extending filtrate channels or compartments spaced about its periphery and covered by a filter screen.
There are two general types of drum filters, the center valve type and the pipe machine or end valve type. In a center valve type filter each filtrate compartment drains into a center circumferential valve opening which communicates through a central axial discharge pipe with a remote source of subatmospheric pressure. An arcuate, stationary valve closure member is mounted at the periphery of and extends partially about the channel. As the drum rotates about its axis with subatmospheric pressure being applied, the screen rotates through the pulp slurry, collecting a wet mat of fibers from the slurry. As the screen emerges from the tank, filtrate is drawn through the screen from the pulp and drains through the axial compartments into the center valve opening, and from there through the axial discharge pipe from the drum, thereby removing the liquid from the mat. As the drum continues to rotate, the stationary valve member periodically blocks off a segment of filtrate compartment outlets from the source of subatmospheric pressure, thus enabling removal of the pulp mat from the surface of the screen. A center valve type rotary drum filter is described in more detail in, for example, U.S. Pat. No. 4,154,687.
Filtrate drawn through the screen and drained into the center valve opening from the axial filtrate compartments tumbles randomly and turbulently under the influence of the subatmospheric pressure and, in upper segments of the drum, gravity, into the central discharge pipe. This random, generally radial flow converges at the opening of the discharge pipe, creating further turbulence. The unrestricted valve opening is never filled with filtrate. The turbulent, random radial flow entraps air and produces resistance to discharge flow and loss of pressure head. There is therefore a need for a more efficient and effective means for promoting filtrate drainage from center valve filter drums than is presently available.
A number of references disclose ways of directing filtrate from the surface of a rotary drum filter to a central drainage conduit. For example, U.S. Pat. No. 2,998,883 to Rich discloses a rotary drum filter which includes a pair of axially disposed collection troughs which distribute filtrate to a central drainage pipe through a pipe extending from the bottom of each channel at an end of the drum. U.S. Pat. No. 3,794,178 to Luthi discloses a rotary drum filter having axially disposed surface channels. Filtrate is fed from each channel to a central drainage conduit through pipes located at the end of the drum. U.S. Pat. No. 3,150,083 to Luthi discloses a valveless rotary drum filter having a large number of axially aligned, radial directed channels which spiral radially inward toward the center of the drum.
None of the drums of the foregoing references are center valve filters or directed to solving the discharge filtrate flow problems of such filters except by, in effect, replacing such filters.
The "Handbook for Pulp and Paper Technologists", G. A. Smook, Canadian Pulp and Paper Association, Montreal, Quebec, 1982, at page 111, discloses an unusual internal center valve type rotary drum filter. Radial vanes provide radial extensions of the usual axial surface channels to channel filtrate to the internal valve. The vanes are apparently required to direct filtrate flow to specific ports of the internal valve to enable the valve to function. In this respect, the filter resembles the pipe machine type. Also, the large number of closely spaced vanes would apparently create substantially more resistance to filtrate flow than is provided by the more usual external center valve filter drums previously described.
While each of the above references discloses ways for directing filtrate flows inward toward the center of a rotary drum filter, none shows or suggests a way of overcoming the inefficient filtrate drainage flow inherent in existing external center valve rotary drum filters between the valve and the central drainage pipe.
Accordingly, it is an object of the present invention to reduce the pressure head loss and flow resistance in the valve opening of an external center valve rotary drum filter.
Another object of the present invention is to accelerate filtrate drainage flow from the surface drainage channels to the central drainage pipe in an external center valve drum filter.
Yet another object of the present invention is to provide a means for increasing the filtrate drainage efficiency of external center valve drum filters.